When plotting accumulated line plots which have NULL values as data (i.e. '-') they will distort the accumulated plot since they will in effect be interpretated as having value 0. This means that there will be a drop in the plot. The only way to avoid this is to calculate the intermediate value.
This can be achieved by calculating a linear interpoaltion for the missing value(s). Calling this method will replace all '-' values with an interpolated value.